This invention relates to the field of rotary machines and particularly to machines that operate efficiently like a DC commutator motor but also operate at fixed selectable speeds over widely varying motor torques and available line voltage.
Rotary machines have generally been classified as either DC or AC machines. Both types of machines include means for generating at least two different magnetic fields, one field being produced in a rotor and the other in a stator. When these fields are not aligned with each other, a torque is established which brings about a rotary movement of the rotor. As the rotor turns, the orientation of the rotor field with respect to the stator field changes. But as the field orientation changes, the magnitude of torque on the rotor also changes so that the machine must include some means for reorienting one magnetic field with respect to the other so as to maintain the torque on the rotor near a maximum in order to sustain efficient machine operation.
In DC machines, the rotor armature normally has a plurality of windings wound thereon which are connected to an external power supply through brushes and a mechanical commutator. The commutator causes power to be applied selectively to the armature windings so that the magnetic field produced in the rotor will be, on the average, aligned at a 90.degree. angle to the stator field produced by fixed stator windings or a permanent magnet. Because the angle between the armature field and the stator field for a DC machine is on the average 90.degree., the DC machine is generally very efficient and torque on the rotor is maximum. However, the DC machine does have a drawback over most AC machines. When the machine is loaded by an external device connected to the rotor, the rotary velocity of the machine decreases in a somewhat linear fashion as the torque load increases. Consequently, DC machinery is not suitable for use in applications where constant speed is required and the load and/or line voltage varies unless some form of auxiliary speed controller is utilized.
AC synchronous machines, on the other hand, are designed to utilize alternating current power to synchronize and power the machine. The fixed frequency of the AC power source is automatically operative to cause rotation at a fixed rotary velocity. As the synchronous machine is loaded, the motor is operative to cause the torque angle, the angle between the rotor field and the stator generated field, to come ever closer to a 90.degree. angle. In this manner, the torque on the rotor increases as the load on the machine increases. Consequently, an AC synchronous machine is able to maintain a constant rotary speed even as the torque load varies. The problem with such synchronous machines is that such machines do not operate at a constant efficiency for all load torques or have the ability to readily adjust motor speed. Indeed, the AC synchronous machine is most efficient when it is heavily loaded because the torque angle approaches 90.degree. as the machine is loaded. When less heavily loaded, the torque angle is less than 90.degree. thus reducing the machine efficiency. AC synchronous machines also have inherent starting problems overcome by many known but somewhat cumbersome techniques.
Machines of other types have also been developed. The brushless DC motor is but one. This type of motor has a rotor in which a magnetic field is produced in a fixed orientation with respect to the rotor. Many stator windings are provided and a control circuit, including an auxiliary transducer to sense rotor position, controls current flow therethrough so as to produce a rotating magnetic field which interacts with the rotor field to thereby produce a torque on the rotor. These machines, however, have generally been designed to meet very specific requirements and are relatively inflexible. That is, the design of a given machine is not necessarily adaptable for use in another machine especially where the other machine is of a different size or of a different number of poles. Most often machines of this type use mechanical transducers to control the machine; such transducers are subject to various types of failure thus giving rise to improper control. Mechanical transducers also fail to consider magnetic variations within the machine which cause physical orientation to be somewhat different from magnetic orientation.
Transducerless machines have been developed but these machines also suffer from the problem of inflexibility found in brushless DC motors.
OBJECTIVES OF THE INVENTION
It is the primary objective of the invention to provide a machine which runs as a DC machine which varies in speed with change in load, as well as having the ability to be synchronized to a selectable reference over a wide range of load torques.
It is a further objective of the invention to provide a machine which produces as great an output and runs as efficiently as a DC commutator machine, has ability to be operated synchronously over a wide range of torques and has a design which is substantially independent of machine size.
It is further objective of the invention to provide a machine which responds to available induced voltage in stator windings to control power pulses applied to the stator windings so that the machine runs characteristically as a DC machine or as a controlled DC machine at constant selectable speeds over a wide range of machine torques.
It is yet another objective of the invention to provide a machine which runs synchronously and efficiently at a selectable rotary speed while the load torques varies by providing means to utilize the zero crossing of induced voltage in each stator winding and ignore the zero crossing of voltage across each stator winding associated with power switching to thereby provide signals indicative of the rotor position for controlling the switching of power from one stator winding to another.
It is another objective of the invention to provide a machine which runs synchronously and efficiently at a selectable rotary speed while the load torque or line voltage varies by providing means to vary the duration of electrical power pulses applied to the motor to compensate for small line voltage and/or load torque variation and providing means to vary the power pulse amplitude applied to the motor to compensate for large line voltage and/or load torque variations.
It is a further objective of the invention to provide means for applying variable width power pulses to stator windings of a motor to maintain a constant average torque angle whereby the motor operates at a constant selectable speed while the load torque varies.
It is yet another objective of the invention to provide a machine which runs efficiently at a selectable constant speed and also is self-starting in a selectable direction of rotation.
It is another objective of the invention to provide a machine which runs synchronously and efficiently at a constant selectable rotary speed by providing means to apply power pulses to each stator winding so that the stator power pulses are generated symmetrically about the time when the torque angle is predetermined value.
It is yet another objective of the invention to provide a machine which runs synchronously and efficiently at a selectable rotary speed by providing means including a pulse generator for producing pulses derived from the zero crossings of the induced voltage in conjunction with a reference oscillator for detecting deviation from the synchronous speed and correcting the machine operation to compensate for the deviation.
It is further objective of the invention to utilize counters to determine the physical position of the rotor of a machine and produce a signal at the time when the physical position is reached to thereby operate the machine efficiently at a variable or at constant selectable rotary speeds.
BRIEF DESCRIPTION OF THE INVENTION
One aspect of the invention is predicated on the concept of utilizing induced voltage in machine stator windings to determine the precise time when pulses should be applied to stator windings so as to maintain constancy of average torque angle with change in speed, resulting from load or line voltage change or constant speed operation over a wide range of machine torques. A further aspect of the invention is predicated on the concept of precisley controlling power pulse frequency, duration and amplitude to maintain constant speed and efficiency as machine torque varies or as line voltage varies.
In a preferred embodiment of the invention, a rotor is provided with a magnetic field therein. The magnetic field in the rotor is fixedly oriented with respect to the rotor. The rotor freely rotates with in a stator located around the rotor, the stator having a plurality of windings arranged so that current flow therethrough will produce a flow which will interact with the rotor field to produce a torque on the rotor causing rotation thereof. As the rotor turns, a voltage is induced into the stator windings. A control network responds to the voltage induced in the stator windings and particularly to the zero crossings of the induced voltage to activate a power network which produces power pulses that are applied to selected stator windings at a predeterminded frequency in a predetermined sequence. These power pulses cause the rotor to rotate at the selected speed in a predetermined direction of rotation. The power pulses are produced at times when the torque angle is on average a selectable value, the selectable value being 90 .degree. when maximum efficiency is desired.
If the rotor speed increases above the selected speed, the control network senses this condition and causes the duration of the power pulses applied to stator windings to be reduced, thus causing the rotor speed to decrease. If the rotor speed decreases below the selected speed, the power pulses applied to the stator windings are widened by the control network, thus increasing rotor speed. The pulse shortening and widening compensates for relatively small speed variations caused by varying load torques on the machine. For larger load torque variations, the control network produces signals which either raise or lower the voltage of the power pulses applied to the stator windings. The increase or decrease in power pulse voltage besides causing the rotor to either speed up or slow down allows the pulse width to remain within selected limits. These power pulse voltage changes are operative to maintain constant speed orientation when large load torque variations occur.
The machine is self-starting in a desired direction of rotation. A self-starting newtork is provided to produce control signals for controlling the power network when the rotor is at rest and no induced voltage is present in the stator windings. The self-starting network directly produces a slowly rotating field in the stator which rotates in the desired direction and causes the rotor to rotate in the desired direction. Once the rotor speed is sufficient to induce a detectable voltage in the stator windings, then the self-starting network is automatically switched off and the control newtork takes over control of the power network.